How to find a speck in space

New Horizons proves stellar parallax can locate a probe in the vastness, using the light of just two stars

- 

New Horizons was the first mission sent to explore Pluto, conducting a fly-by in July 2015. It was then redirected for a close pass of a much smaller Kuiper Belt object, Arrokoth, in January 2019. Today, it's over 61 AU away - almost twice as distant as Pluto - and racing from the Sun at around 50,000km/h (31,000mph), meaning that it covers the equivalent of the Earth-Sun distance every four months.

It's the fifth robotic spacecraft that we humans have launched on an escape trajectory that will ultimately leave the Solar System altogether and enter interstellar space. Indeed, the craft is already so remote that nearby stars appear to shift in position when viewed from the spacecraft, compared to how we see them from Earth. This displacement in the apparent position of objects seen from two vantage points is known as parallax. You can create this effect for yourself with a simple experiment: hold a finger at arm's length and alternate closing each eye. The finger appears to jump back and forth against the more distant background, because each eye sees it from a slightly different angle.

Tod Lauer, at the NSF National Optical-Infrared Astronomy Research Laboratory in Tucson, Arizona, led a team of 53 coauthors from 46 institutions to demonstrate stellar parallax between New Horizons and Earth. They did this by coordinating the probe to photograph two nearby stars with images taken simultaneously from ground-based observatories.